Mishkat al-Masabih
Mishkat al-Masabih 3398
وَعَن سفينة قَالَ: كُنْتُ مَمْلُوكًا لِأُمِّ سَلَمَةَ فَقَالَتْ: أُعْتِقُكَ وَأَشْتَرِطُ عَلَيْكَ أَنْ تَخْدُمَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مَا عِشْتَ فَقُلْتُ: إِنْ لَمْ تَشْتَرِطِي عَلَيَّ مَا فَارَقْتُ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مَا عِشْتُ فَأَعْتَقَتْنِي وَاشْتَرَطَتْ عَلَيَّ. رَوَاهُ أَبُو دَاوُدَ وَابْنُ مَاجَهْ
Safinah reported: I was a slave belonging to Umm Salamah, and she said, ‘I will emancipate you on the condition that you serve the Messenger of God as long as he lives.’ I replied, ‘Even if you did not make that condition, I would not leave the Messenger of God as long as he lives.’ So she emancipated me and made that condition. Abu Dawud and Ibn Majah transmitted it.